THE INTERPRETER IO3 



ceived from and through its respective parent, and 

 must therefore be of inconceivable complexity. We 

 can only speak in generalities of processes of which 

 so little is known ; but we cannot be wrong in as- 

 suming that sterility is sometimes due to the fact that 

 the complexity of the one part-nucleus fails in some 

 way to suit the complexity of the other. 

 The length of time required for mutual sterility to be 

 complete may be inferred from the fact that entirely 

 distinct, but closely related, species are still partially 

 fertile in that they can produce hybrid offspring. 

 When our domestic breeds of pigeons have been en- 

 tirely prevented from interbreeding for some immense 

 period of time, we may expect that they too will only 

 produce sterile hybrids, and, later still, not even these. 

 At present the majority of these breeds are not every- 

 where rigidly prevented from interbreeding, so that an 

 approximation to natural species-formation has not 

 even begun. There are others, however, such as the 

 most widely different breeds of dogs, in which the di- 

 vergence in size is so extreme that interbreeding has 

 probably been a mechanical impossibility for some 

 considerable time. 1 



Huxley's letters express dissent from the so-called 

 " Neo-Lamarckian " school, represented by Mr. 

 Herbert Spencer, which contends that the use and 

 disuse of organs, together with the action of sur- 

 roundings, produce modifications of structure which 

 are transmitted to offspring. There are no specific 

 references to Professor Weismann's Essays in Heredity 

 (1883), in which, representing the so-called " Neo- 

 Darwinian " school, natural selection, acting on 



1 January, 1 90 1, pp. 269, 271. 



